1. Field of the Invention
The invention relates to interlaced video display, and more particularly to an image sensor for displaying interlaced video using optical summing.
2. Description of the Related Art
In full color video applications, video cameras typically employ the CMYG (Cyan, Magenta, Yellow, and Green) filter pattern for capturing full color video images. The CMYG filter scheme is preferred over the traditional RGB (Red, Green, Blue) primary color scheme because the CMYG filters have higher transparency (which results in higher sensitivity) and because the use of CMYG colors simplifies post-capture image processing. FIG. 1 illustrates a typical CMYG filter pattern used in conventional video cameras. CMYG filter pattern 50 includes alternating rows of cyan/yellow filters (rows 51 and 53) and green/magenta filters (rows 52 and 54). In the second row of green/magenta filters (row 54), the positions of the green and magenta filters are switched so that a magenta filter is positioned below a cyan filter of the row above and a green filter is positioned below a yellow filter of the row above. The 4 by 4 block of filter pattern 50 is repeated to cover the entire array of pixel elements in the image sensor of the video camera.
Video cameras output video data in an interlaced format because most display monitors display video data in an interlaced format. That is, video data are outputted by alternating between two data fields, an odd field and an even field, every 1/60th of a second. Video data associated with the odd rows of the image sensor are outputted in the odd field while video data associated with the even rows of the image sensor are outputted in the even field. The odd and even data fields are thus interlaced. Each of the odd and even data fields has a resolution that is half the vertical resolution of the image sensor. For example, if the image sensor has a resolution of 720 columns by 480 rows of pixel elements, each data field output a video image having a resolution of 720 by 240.
To generate interlaced video data, the entire field of the image sensor is used. In operation, the image sensor captures a frame of image data and pixels values in adjacent rows of the image sensor are summed to generate the video data for either the odd or the even data field. Then, the image sensor captures another frame of image data and pixel values in the alternate, adjacent rows of the image sensor are summed to generate the video data for the other data field.
FIG. 2 illustrates the summing operation for generating the interlaced video data in an image sensor overlaid with the CMYG filter pattern of FIG. 1. The video data capture scheme is referred to as the CMYG interlaced capture scheme. Referring to FIG. 2, image sensor 60 is illustrated as having 8 rows and 8 columns of pixel elements. To generate the odd field image, pixel data in adjacent pixel elements in rows 1 and 2, rows 3 and 4, rows 5 and 6 and rows 7 and 8 are summed, resulting in four rows of interlaced video data. For the even filed image, pixel data in adjacent pixel elements in rows 2 and 3, rows 4 and 5, and rows 6 and 7 are summed, resulting in another four rows of interlaced video data.
Because video output signals are typically given in the YCbCr color space, post-capture image processing is performed to convert the pixel data captured in the CMYG color space to the YCbCr color space. (In the following description, the italic letter Y is used to denote the luminance channel in the YCbCr color space while the letter Y is used to denote the color yellow in the CMYG color space.) Color interpolation from CMYG to YCbCr is well known. FIG. 3 illustrates the interpolation operation involved in converting the pixel data in CMYG color space to the YCbCr color space.
Referring to FIG. 3, the luminance channel Y is estimated by adding pixel values in adjacent columns of pixels. For example, for the odd field, the luminance channel Y for the first row is the sum of (C+G) and (Y+M) while the luminance channel Y in the second row is the sum of (C+M) and (Y+G). The chrominance channels Cb and Cr are estimated by subtracting pixel values in adjacent columns of pixels, as shown in FIG. 3. The chrominance channel Cr is the difference of (Y+M) and (C+G) while the chrominance channel Cb is the difference of (C+M) and (Y+G). In this manner, the luminance channel Y is estimated at the intersection of every pixel while the chrominance channels Cb and Cr are estimated at every other pixel to form the desired color demosaic. The color interpolation for the even field operates in the same manner as in the odd field. The even field has a different color filter ordering but the same combination of rows.
The use of the CMYG interlaced capture scheme has several advantages over the traditional RGB capture scheme. First, the CMYG color filters, being complementary colors and spanning a broader spectrum of light frequencies, are capable of capturing more light than RGB filters. Thus, the sensitivity of the image sensor can be enhanced. Second, post-capture image processing is simplified. By using the CMYG filter pattern, video data output in the desired YCbCr color space can be obtained readily. Specifically, only one adder is needed to interpolate the luminance Y channel and only one subtractor is need to interpolate either of the chrominance channel Cb or Cr. Thus, the interpolation operation is computationally simple and can be performed at a very fast speed. High-speed interpolation is particularly important for video images as the output rate is one data field per 1/60th of a second.
Finally, by summing pixel values of adjacent rows, the image sensor's sensitivity can be increased. This is true for image sensors where pixel data are measured in analog form and converted to digital values by circuits outside of the image sensor array, such as CCD or APS image sensors. In an CCD image sensor, charge accumulates in each pixel element. The summing of the analog pixel values has the effect of increasing the photo-sensitive area of the pixel cell, albeit under two different color filters. By summing the analog pixel values of adjacent pixels and then digitizing the summed value, inaccuracy due to quantization noise can be avoided.
However, when the image sensor in the video camera is one in which pixel-level analog-to-digital conversion (ADC) is performed, the summing operation has to be performed using the digitized pixel values. Summing digitized pixel values has several disadvantages. First, the summing operation will become more computationally complex. The digital values have to be added using a data processor. Second, because of quantization noise in the digitized signals, the effective sensitivity of the image sensor is decreased.
Therefore, it is desirable to provide an alternate implementation of the CMYG interlace capture scheme in an image sensor to avoid the aforementioned disadvantages.